1. Field of the Invention
The invention relates to a single-mode rotary joint for optical signals, having two channels, with which the signals can be simultaneously transmitted in opposite directions.
2. Description of the Prior Art
Because of their greater flexibility and robustness, optical bus systems are frequently used instead of cable-bound bus systems. With bus systems of this kind, signals or information are regularly transmitted in two opposite directions of the bus system, in order to make possible a bidirectional communication between different subscribers.
Optical rotary joints are known for transmission of optical signals between units that are rotatable relative to each other.
An optical rotary joint for a plurality of channels, having a Dove prism, is disclosed in U.S. Pat. No. 5,568,578. With a rotary joint of this kind, substantially more than two channels may be transmitted. It thus offers an excellent flexibility. However, the high costs of the elaborate mechanical arrangement render an optical joint of this kind of no interest for many applications.
A rotary joint having two channels is disclosed in U.S. Pat. No. 5,588,077. In all embodiments, one beam path (channel B) is widened by a pair of lenses, and then again narrowed, the optical elements of the other beam path (channel A) being disposed therein. Complex lens systems are needed with this device in which, in particular, a widening of the channel B must be effected to the extent that the optical elements of the channel A have an only insignificant effect. Furthermore, this device cannot be constructed to have rotational symmetry, because at least two light guides must be inserted into the beam path radially from the outside. This non-symmetry leads to an attenuation that is dependent upon an angle of rotation.
Another two-channel rotary joint is disclosed in DE 20018842. With this, light is coupled from a light-guiding fiber disposed to be inclined to the axis of rotation into another light-guiding fiber which is rotatable relative thereto and disposed on the rotation axis. Another corresponding coupling means is provided for the opposite beam path. A disadvantage of this arrangement is that the attenuation is very large with single-mode fibers.